Known in the art is an apparatus for forming films by evaporation in vacuum (GB, B, 1517085), comprising a debiteuse containing a material being evaporated, the electric current flowing directly through the debiteuse for heating and evaporating a coating material. A substrate is placed horizontally opposite to the debiteuse and is spaced therefrom.
This apparatus is characterized by large losses of a coating material because of the absence of a directional flow of the evaporated material, and it can only be used for forming films on horizontal substrates.
For creating a directional flow of an evaporated material, nozzles are used, e.g. in an evaporator for vacuum plants (SU, A, 397567) having a heater accommodating a crucible containing a material to be evaporated and a nozzle provided in the top part of the crucible. A substrate is horizontally positioned and is aligned with the nozzle axis and spaced from the outlet edge of the nozzle.
This prior art evaporator also features availability of a boundary layer in the flow adjacent to the walls of the nozzle, the width of the boundary layer covering a substantial part of the flow with velocities of evaporated materials used in practice. Molecules emitted from this layer move chaotically and cause an increase in losses of a coating material.
A substantial reduction of losses of a coating material can be achieved by using the technique of solid phase recovery of an evaporated material that failed to get to the substrate as it is the case in an apparatus for forming films by evaporation in vacuum (U.S. Pat. No. 4,700,660), comprising a chamber for evaporating a coating material, a heating element provided outside the chamber, and a collimating chamber communicating therewith, each chamber of identical construction having two ports aligned on one and the same axis.
The chamber for evaporating a coating material and the collimating chamber comprise a pipeline having a transverse plane of symmetry. The bottom end of the evaporation chamber is positioned in a crucible containing a coating material. Both chambers may be separated by a diaphragm with an opening extending in the plane of symmetry.
The pipeline is turned over at regular intervals as the coating material is consumed and replenished and is placed in such a manner that it should be received in the crucible with an end thereof on which the coating material that failed to get to the substrate has been deposited. This material is used in the next evaporation cycle.
The abovedescribed apparatus is, however, suitable for forming films only on horizontally positioned substrates as the coating material can flow out through unsealed spots in the joint between the bottom end of the pipeline defining the evaporation chamber and the crucible. In addition, in the simplest versions of the apparatus in which the pipeline is of an integral construction and has an annular recess in the middle part separating the evaporation and collimating chambers, the material is always condensed in the annular recess zone with any position of the pipeline. The coating material collected in the annular recess would overflow to one of the chambers so as to cause an asymmetric change in distribution of intensity of vapour flow escaping from the collimating chamber thus lowering uniformity of thickness of the applied films.
In addition, this apparatus in its simplest form cannot ensure high uniformity of thickness of the applied film on large-area substrates even if they are positioned horizontally, and the use of various diaphragms provided for that purpose in the pipeline complicates construction of the apparatus.
In case rather thick films are to be formed, a layer of material condensed in the collimating chamber causes a change in distribution of intensity of vapour flow escaping therefrom thus lowering uniformity of film thickness.
Moreover, evaporation of the coating material simultaneously from the crucible and evaporation chamber which are generally at somewhat different temperatures results in a change in evaporation rate during evaporation: first evaporation of the material occurs at a higher rate from a stronger heated surface (evaporation chamber) and then the material evaporates at a lower rate from a less heated surface (crucible). For ensuring a parity between evaporation rates, it is necessary to control heating current during evaporation which makes the apparatus more complicated.